Such an NMR-MAS probehead and the associated coil block are known from patent DE 10 2016 207 998 B3 (=reference [1]).
In general, the present invention relates to the field of magnetic resonance (MR). Nuclear magnetic resonance (NMR) spectroscopy is a commercially widespread method in MR for characterizing the chemical composition of substances. In MR, the measurement sample which is situated in a strong static magnetic field is generally irradiated by radiofrequency (RF) pulses and the electromagnetic reaction of the sample is measured. Further, it is known in solid-state NMR spectroscopy to rotate an NMR sample tilted at the so-called “magic angle” of approximately 54.74° in relation to the static magnetic field during the spectroscopic measurement (“MAS”=Magic Angle Spinning) in order to minimize line broadening on account of anisotropic interactions. To this end, the sample is inserted into a MAS rotor. MAS rotors are cylindrical tubes which are sealed with one or two caps, the upper one being provided with blade elements (“impeller”). The MAS rotor is arranged in an MAS stator and the MAS rotor is driven for the purposes of the rotation by gas pressure by way of the blade elements. The totality of MAS rotor and MAS stator is referred to as MAS turbine.
The MAS turbine is arranged in an NMR-MAS probehead during the NMR measurement. The probehead comprises a cylindrical shielding tube. Housed therein are RF electronic components, in particular RF coils, and the MAS turbine. With the shielding tube thereof, the probehead is typically inserted from below into the vertical room temperature bore of a superconducting magnet, positioned therein and held therein with hooks, supports, screws or the like. The MAS turbine is then situated precisely in the magnetic center of the magnet.
In addition to solid-state NMR, use may also be made of the dynamic transfer of spin polarization (DNP=dynamic nuclear polarization) technique. The DNP technique requires simultaneous irradiation of a magnetic microwave field at a frequency which is higher than the Larmor frequency of the 1H nuclei by a factor of approximately 660. Electron spins are excited by irradiation of a microwave field at a suitable frequency, whereupon a transfer of electron polarization onto the atomic nuclei of the sample may be brought about on account of spin interactions.
Currently, in the prior art, the microwave in DNP probeheads is irradiated by virtue of the port of a corrugated waveguide (double miter bend) being directed onto the MAS rotor. Although this approach is easy to implement from a technical point of view, only a fraction of the employed power is used. The reason for this lies in unwanted reflections, diffractions, etc. Moreover, in systems with rotor diameters of 1.3 mm (more accurately: just below 1.3 mm), an RF coil with a lower efficiency in comparison with non-DNP probeheads is used as it would otherwise hardly be transmissive in relation to microwaves.
In reference [1] cited at the outset, the predecessor project in relation to the present invention, the applicant developed an optimization which, inter alia, approximately doubles the microwave magnetic field in the DNP stator with a rotor bore of 3.2 mm with the aid of a lens and a mirror, reducing the power for the same field by a factor of 4.
If smaller MAS diameters are considered, it is easy to estimate that focusing on a smaller cross section should lead to significant field increases; the power savings lie at approximately a factor of 5 for a cross section of the MAS rotor of approximately 1.9 mm and approximately 20 for a rotor diameter of approximately 1.3 mm.